Most golf balls in use today are manufactured by injection molding or compression molding a material composed largely of urethane resin, ionomer resin or the like over a solid core that typically consists primarily of rubber such as a diene rubber.
The above resins generally deteriorate under the action of heat, light, oxygen, mechanical shear forces, ozone, NOx gases and the like, sometimes giving rise to undesirable effects such as a decline in physical properties and a poor appearance due to discoloration. Various types of inhibitors are used to suppress such effects. In particular, thermal antioxidants are used to inhibit deterioration by heat. Such antioxidants include radical scavengers (primary agent) and peroxide decomposers (secondary agent), the primary agent being exemplified by phenolic antioxidants and the secondary agent being exemplified by phosphate antioxidants and sulfur antioxidants.
Polymers typically undergo deterioration when exposed to heat or light or upon contact with oxygen. In particular, ultraviolet light having a wavelength of 400 nm or less possesses a lot of energy and acts directly on polymers, destroying their bonds. Furthermore, in the presence of oxygen, the oxidative deterioration of polymers is also promoted. These actions can generally be suppressed by the addition of ultraviolet absorber and light stabilizer (e.g., hindered amine light stabilizers, abbreviated as HALS) ingredients. By using both in combination, or by additionally using also a thermal antioxidant, synergistic effects can be obtained.
The cost of golf balls can be lowered by mass production, but a large amount of scrap such as runners is generated, in proportion with the production volume, when injection-molding covers and the like. Normally, such scrap is granulated and then reused as reclaimed material by blending the scrap in a predetermined proportion with virgin material. However, because the reclaimed material has already incurred a thermal history from molding and the like, there is a concern that, if blended in too large an amount, it may have a deleterious effect on the appearance and durability of the molded articles. Hence, at present, from the standpoint of maintaining the quality of the molded articles, there is a limit to the amount of reclaimed materials that can be used. It is generally recommended that the amount of such reclaimed material be set to not more than about 23 wt % of the overall molding material (that is, about 30 parts by weight per 100 parts by weight of virgin material). Accordingly, the development of efficient manufacturing methods capable of facilitating the use of reclaimed materials while maintaining the quality of the molded articles is important both from an environmental standpoint and also from an economic standpoint.
Prior-art documents relating to this invention include the following: JP-A 2000-178437, JP-A 2001-303369, JP-A 2003-166181, JP-A 2003-238796, JP-A 2004-182980, JP-A 2004-346146, JP-A 2007-217678, JP-A 2007-144097, JP-A 2008-504422 and JP-A 2009-66241.